Cognitive radio is attracting attention as a technique to handle an increase in traffic in wireless communication. There is a form of the cognitive radio that is currently being considered in which a secondary system performs a secondary use of a frequency channel (a white space) that is not used spatially and temporally by a licensed primary system in the range not influencing services operated by the primary system. As a related art concerning such an operation of the secondary use of the white spaces, there is a method disclosed in Non Patent Literature 1. In this method, for a position of a transmission station of a secondary system (a secondary transmission station) that performs the secondary use, a list of channels available for the secondary use at the position is stored to a database according to categories explained below. Further, by referring to the database at the time of performing the secondary use, the secondary transmission station obtains channels available for the secondary use at the position. Specifically, a service area of a primary system which should be protected is calculated as a protection zone by a predetermined propagation prediction method, and a required value of a separation distance from a boundary of the protection zone to the secondary transmission station is set according to the categories explained below. The categories of the secondary transmission station are: frequency channels for secondary use (channels the same as or adjacent to the primary system); a type of a secondary system transmitter (personal use, portable device, or fixed device); and an installation height of a transmission antenna (less than three meters, three meters or greater and less than ten meters, or ten meters or greater and less than 30 meters). An evaluation of whether or not the secondary use is allowed is made depending on whether the separation distance that is necessary according to these categories can be obtained at the position of the secondary transmission station. Results of the evaluations are stored to the database.
There are two problems with this above technique. The first problem is that the evaluation of whether or not the secondary use is allowed is based on the separation distance from an end of the protection zone, while terrain and feature of a propagation path from the secondary transmission station to a reception station of the primary system (a primary reception station) is not taken into account. The second problem is that an increase in a level of interference from the secondary use by a plurality of secondary transmission stations in the same time period is provided as a fixed margin, while the actual number of simultaneous secondary transmission stations is not taken into account. As a result, it has been highly possible to cause harmful interference on the primary reception station from the secondary use of the white space by the secondary transmission stations or miss a chance for the secondary use by the secondary transmission stations by maintaining a separation distance greater than necessary.
As a technique to solve the first problem, there is a method disclosed in Non Patent Literature 2. This method evaluates whether or not to allow the secondary use of channels based on a propagation prediction by using site-specific information between the secondary transmission station and the primary reception station so as to estimate the level of interference on the primary reception station from the secondary transmission station performing the secondary use, that should be considered. Accordingly, this method predicts the level of interference with higher accuracy than when the prediction is made only based on the separation distance. Therefore, it is possible to reduce the probability of creating harmful interference on the primary system and maintaining a separation distance greater than necessary.
As a technique to solve the second problem, there is a method disclosed in Patent Literature 1. This method obtains the number of secondary transmission stations that perform the secondary use in the same time period, predicts the total level of interference on the primary reception station when these secondary transmission stations start the secondary use, and confirms that the transmission power is configured not to exceed an allowable interference level before starting the secondary use. Thus, it is possible to reduce the possibility of creating harmful interference on the primary system or maintaining a separation distance greater than necessary.